Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for retaining pads within a bearing device.
Turbo machinery is evolving and the latest technology in this field is making use of high speed bearings. In many areas of bearing design, the design of the rotor-bearing system directly influences the performance of the machine. Traditional designs have provided bearings with rolling elements, i.e., pads or shoes that may pivot around a retaining head while supporting a rotor. However, at high speeds and/or high pressures, the load capacities and stiffness limits of the rolling elements are exceeded and thus, the performance and life expectancy of the machinery is reduced. For example, at peripheral speeds above the typical speed for a traditional turbo machinery, ball-bearings placed at the ends of a rotating shaft to accommodate bearing speed limits may lead to super critical (i.e., operating above critical speed) rotor design. In turn, the super critical rotor design may result in an unstable rotor which may be subject to destructive and unpreventable subsynchronous whirl and to large radial deflections.
While conventional applications of a turbo machinery employ a traditional peripheral speed, it appears that a machine that can operate at higher speeds would improve power consumption and also heat distribution in pads, among other advantages. However, these higher-than-normal speeds may contribute to other problems that are discussed next.
In order to reduce friction between a rotor and a bearing, oil may be introduced to separate the two components of the machine. In the art, this system is known as a journal bearing. The shaft and bearing are generally both simple polished cylinders with lubricant filling a gap between the shaft end and the shoes of the bearing. Rather than the lubricant just “reducing friction” between the surfaces of the shaft and the shoes, letting one slide more easily against the other, the lubricant is thick enough that, once rotating, the surfaces do not come in contact at all. If oil is used, it is generally fed into a hole in the bearing under pressure, as is done for loaded bearings.
Such an example is shown in FIG. 1, which is an illustration of FIG. 2 of U.S. Pat. No. 4,568,204, the entire content of which is incorporated herein by reference. FIG. 1 shows the journal bearing 10 enclosing a shaft 12 that rotates as shown by arrow 14. Journal bearing 10 includes five pads 16 that are retained in place by a ring 18. Each pad 16 includes a pad support 20 inserted into a recess region 22 of the pad 16. The pad support 20 is connected to an insert 24 that is fixed to the ring 18. Each pad support 20 and recess region 22 have cooperating spherical surfaces to allow the pad 16 to pivot freely in any direction to conform to the surface of the shaft 12 as it rotates. In addition, the journal bearing 10 has oil supply devices 26 regularly formed inside the ring 18 for supplying the oil between pads 16 and shaft 12.
However, when shaft 12 rotates relative to the pads 16 with a speed of 80 m/s, oil starvation is one of the problems found in traditional turbo machinery. This problem is exacerbated when the peripheral shaft speed is increased. Oil starvation is the lack of enough oil for the rotating shaft and/or pads such that the oil film between the shaft and the pads is interrupted, which may lead to high friction between the shaft and pads, leading to high temperature and subsequent damage.
Another problem that may appear in traditional turbo machinery is cavitation. Cavitation is the formation of vapor bubbles of a flowing liquid (oil for example) in a region where the pressure of the liquid falls below its vapor pressure. As the rotor rotates with a high speed, the pressure of the oil may fall below its vapor pressure, leading to cavitation and formation of shock waves. Since the shock waves formed by cavitation are strong enough to significantly damage moving parts, cavitation is usually an undesirable phenomenon.
As the peripheral shaft speed of new applications require speeds in excess of 170 m/s, the above summarized problems have to be addressed and solved in order for these applications to function appropriately. While the above problems have been discussed in the context of journal bearings, these problems are found in other bearings in which the shaft rotates relative to the pads at high speeds.
Accordingly, it would be desirable to provide devices, systems and methods that avoid the afore-described problems and drawbacks as well as others understood by those of ordinary skill after consideration of the subject matter disclosed below.